1. Field of the Invention
The present invention relates to a technique for controlling the rotation of a rotor, and particularly to a motor driving circuit which controls the rotation of a motor including a stator having multiple coils and a magnetic stator.
2. Description of the Related Art
In electronic devices employing disk-type media such as portable CD (Compact Disc) devices, DVD (Digital Versatile Disc) devices, etc., a brushless DC motor is employed for rotating the disk. In general, the brushless DC motor includes a rotor including a permanent magnet and a stator including multiple star-connected phase coils. With such an arrangement, in general, the coils are magnetized by controlling the current to be supplied to the coils, so as to relatively rotate the rotor with respect to the stator, thereby driving the brushless DC motor. In order to detect the rotational position of the rotor, in general, the brushless DC motor includes a sensor such as a Hall element, an optical encoder, or the like. With such an arrangement, the current to be supplied to each phase coil is switched according to the position detected by the sensor, thereby providing suitable torque to the rotor.
In order to provide a smaller-size motor, a sensor-less motor has been proposed, having a function of detecting the rotational position of the rotor without involving a sensor such as a Hall element (see Patent documents 1 and 2, for example). With such a sensor-less motor, for example, the electric potential at a common center tap in the motor (which will be referred to as the “center tap voltage” hereafter) is measured. Furthermore, the back electromotive force (induced voltage), which occurs in a coil, is monitored. Moreover, the position information is obtained by detecting the zero-crossing point at which the back electromotive force is equal to the center tap voltage.
In order to monitor the back electromotive force of a phase coil in the driving of such a sensor-less motor, there is a need to set the non-driving state at a timing of the zero-crossing point. In the non-driving state, a driver connected to the phase coil stops the switching driving operation, thereby setting the high-impedance state. In particular, in the driving of a sensor-less motor employing a 180 degree conduction method, the current flows through the coils in all the stages. Accordingly, such an arrangement needs to predict the timing at which the zero-crossing point occurs, and needs to set the high-impedance state based upon the prediction results.
1. For example, Patent document 5 discloses a method for providing a timing of setting the high-impedance state using the PLL (Phase Locked Loop) method.
2. Also, a related technique is described in Patent document 3.
3. Also, a technique is known in which the current passing through a phase coil is smoothly controlled in the shape of a sine wave or an arch-shaped waveform (see Patent documents 3 and 4, for example).
[Patent Document 1]
    Japanese Patent Application Laid-Open No. Hei3-207250[Patent Document 2]    Japanese Patent Application Laid-Open No. Hei10-243685[Patent Document 3]    Japanese Patent Application Laid-Open No. Hei11-75388[Patent Document 4]    Japanese Patent Application Laid-Open No. Hei11-341870[Patent Document 5]    Japanese Patent Application Laid Open No. 2001-190085
Let us consider an arrangement in which the setting of the non-driving state is employed, and the non-driving period, in which the coil is set to the high-impedance state, is set to a long period of time. Such an arrangement has the advantage of detecting the zero-crossing point in a sure manner. However, this leads to the current flowing through the motor in a discontinuous waveform, resulting in increased noise produced by the motor. Accordingly, it is desirable that the non-driving period is reduced as much as possible. However, with such an arrangement that employs an extremely short non-driving period, in some cases, undesirable situations can occur, such as a situation in which the zero-crossing point does not match the timing of the non-driving period. Such a situation thus occurring leads to irregularities in the rotation of the motor. At worst, this leads to a problem of motor stoppage. Accordingly, there is a need to adjust and set the non-driving period according to the revolution of the motor and the change of the load.
Also, in order to continuously change the current that passes through the coil of the motor in the shape of a sine wave or an arch-shaped waveform, an arrangement is known in which the voltage to be applied to the coil of the motor is controlled by means of pulse modulation. FIGS. 7A through 7C are time charts which show the detection of the zero-crossing point in such an arrangement employing a pulse modulation driving method. FIG. 7A is a waveform diagram which shows a pulse modulated signal PWM. FIG. 7B is a waveform diagram which shows a phase voltage (which will also be referred to as the “back electromotive force Vu” hereafter) occurring at the coil, which is a detection target for the zero-crossing point, and a center tap voltage Vcom. FIG. 7C is a waveform diagram which shows a back electromotive force detection signal BEMF_EDGE.
As shown in FIG. 7B, noise components occur in the back electromotive force Vu that occurs in the coil, which is the detection target for the zero-crossing point, at the timing of the transition from the OFF state to the ON state of the pulse modulated signal shown in FIG. 7A, or the timing of the transition from the ON state to the OFF state. Such a noise component repeatedly switches the back electromotive force detection signal, which is obtained by making a comparison between the phase voltage Vu and the center tap voltage Vcom, between the high-level state and the low-level state. This leads to false detection of the zero-crossing point. A false detection of the zero-crossing point is equivalent to an error in detection of the position of the rotor. Accordingly, this leads to a problem of poor rotation precision, rotational malfunction, etc.
3. Furthermore, in order to control the current that passes through the phase coil using the PWM method, there is a need to control the driving current in the shape of a sine wave synchronously with the rotation state of the motor, i.e., the positional relation between the rotor and the stator. In a case in which the driving current is not controlled synchronously with the rotational state of the motor, problems occur such as a problem of noise occurring, a problem of motor rotation stoppage, etc.